1. Field of Invention
This invention relates to a system and method for designing ultrasonic transducers and, in particular, to a system and method for determining optimal values for the geometrical features of an ultrasonic transducer having one or more elements.
2. Description of the Background Art
Ultrasonic transducers containing one or more independently controlled transducer elements are commonly used in the fields of non-destructive testing, medical imaging, medical therapy, flow meters and sonar. Such transducers may be used in conjunction with one or more acoustic lenses and/or one or more acoustic mirrors to achieve a desired ultrasonic insonification. No single transducer design can meet the needs of all the applications of ultrasound, even when such applications are categorized in a single field. For this reason, it is commonplace to design ultrasonic transducers, along with any associated lenses or mirrors, specifically for each application.
Classical single element ultrasonic transducers can usually be adequately specified using a small number of parameters, typically limited to the transducer's diameter, center frequency, and focusing distance. These parameters are often straightforward to determine based on the desired beam spot size in the material to be insonified, the desired focusing location in the material, the ultrasonic velocity in the material, the standoff distance from the surface of the material, if any, and the ultrasonic velocity in the coupling medium, if any.
In contrast, modern array transducers contain several elements. Their specification requires additional parameters including the number of elements and the size and shape of each element. Instead of a single focal point, an array transducer can generate a variety of focal points in different directions, and can adapt to varying material properties and geometries, making the determination of the transducer parameters more difficult.
Modern ultrasonic transducer manufacturing techniques, such as those based on piezocomposite technology, allow arbitrarily shaped conventional and array transducers to be manufactured to high accuracy relative to the ultrasonic wavelength. This added flexibility has further increased the complexity of specifying the parameters necessary for transducer manufacture.
In response to these new challenges in ultrasonic transducer design, approaches and tools have emerged in the relevant art to assist in the transducer development process. For example, several commercially available software packages exist which can be used to numerically simulate the ultrasonic energy radiated by candidate transducer designs. Such software packages are highly valuable to verify that the proposed transducer meets all the performance criteria before the considerable expense of transducer manufacture is undertaken.
The paper entitled “Ultrasonic Phased Array Inspection of Titanium Billets” (Lupien, Vincent and Cancre, Fabrice, Review of Progress in Quantitative Nondestructive Evaluation, Vol. 20, Edited by Thompson and Chimenti, AIP Conference Proceedings, 2001), listing the applicant as a co-author, hereof presents a method to establish the ring and sector boundaries of an annular-sectorial transducer array used for the inspection of titanium cylinders used in the aircraft engine industry. The shape of the array is prescribed to be that which produces spherical focusing at the deepest focusing point inside the cylinder. This paper is incorporated herein by this reference.
The paper entitled “High Sensitivity Inspection of Titanium Forgings in the ETC Program: Probe Design and Implementation” (Roberts, Ron, Phased Array Ultrasound for Aerospace Applications Workshop, University of Dayton Research Institute, 2004), presents a custom designed ultrasonic array transducer for use with an ultrasonic mirror. The ultrasonic transducer is targeted to engine disk inspection and makes use of a more advanced shape composed of three concentric regions with distinct radii of curvature. The three curvature design was proposed to help reduce the number of rings on the transducer.
In prior art such as the papers cited above, the transducer design must be performed by a skilled expert based on physical knowledge, judgment, intuition and considerations of practical issues, along with verification that the design is valid through repeated numerical modeling, numerical simulations and laboratory experimentation. This design process is both time consuming and lacking in consistency. A great number of distinct valid transducer designs can exist other than the ones proposed by the designers. The specific transducer designs achieved depend on the talent of the designer and the ad hoc design method used. The transducer design cannot be ascertained to be optimal in any real sense. For example, other valid designs may exist which require fewer transducer elements. Since the cost of the probes and driving electronics both increase with the number of elements in the transducer, it is usually desirable to achieve a transducer design that minimizes the number of elements. Alternatively, space constrained ultrasonic applications may benefit from the minimization of the size of the transducer.
While the state of the art provides means to achieve designs which are adequate, what is lacking is a more scientific method of ensuring that optimal transducer designs are obtained and can achieve more consistent results.